I'd have to agree here
Even during the Republican period the Italian allies were regarded as better
cavalry troops than the Romans themselves and the Italian allies didn't match up to the Numidians, Iberians, Gauls, Germans etc, hell, anybody who knew 1 end of a horse from another. If the Romans are used as the basis for Imperials then the Imperials have dreadful cavalry
edit: And if we're talking post-Republican than as paw-prints says we're talking auxilleries, not Roman at all really. Byzantine cavalry were quite good but they were mostly recruited from barbarians
Early on, Roman cavalry did take a backseat, but at the same time, it was more effective to maintain a large professional infantry army, as horses weren't cheap, they'd either have to give increased salary, or pay for it themselves. Not cheap when you're dealing with tens of thousands of troops. Cavalry was initially reserved for the upper classes that could afford it.
But they weren't stupid. After they learned of the effectiveness of cavalry (the hard way), it became more and more important and effective. I fail to see how they were "dreadful." At least in the late empire.
Also, Imperials aren't a carbon-copy of the Romans. If they were, then some parts of Tamriel would still be unconquered.